When a blood clot forms in a venous vessel and is left in the vessel for more than about two weeks, it starts forming fibrin strands, so-called synechiae. The fibrin strands do not only contribute to an obstruction of the vessel, but also thicken the vessel walls and thus limit the elasticity of the vessel walls.
It is known to reopen a body vessel by implanting a stent that presses the fibrin strands against the vessel wall. It is further known to remove fibrin strands from vessel walls with fairly complex tools.